Sulfur speciation in volcanic gases is a potentially
valuable tracer of degassing processes at volcanoes.
Hitherto, observations of sulfur speciation in volcanic gas
plumes have however been limited both in number and
quality. Here, we report on periodic measurements of SO2 to
H2S proportions in the volcanic gases from La Fossa
volcano (Vulcano Island) performed during 2004–2006, a
period which encompasses two heating events of the
fumarolic field in January–April 2005 and December
2005. Results indicate a systematic relative increase (by a
factor of 2–6) of SO2 to H2S proportions in the fumaroles
during the heating events, which we ascribe to a
temperature increase in the mixing zone between
magmatic and hydrothermal fluids. We also demonstrate
that sulphur speciation in La Fossa fumaroles reflects
re-equilibration within a poly-baric hydrothermal system,
and that this hydrothermal re-equilibration erases the pristine
SO2/H2S ratios of any magma-derived sulphur present.